1. Field of the Invention
The present invention relates to a method for attaching a substrate such as a semiconductor wafer to a supporting plate when the substrate is thinned by grinding.
2. Description of the Prior Art
There has been a need of making IC cards for cell phones thinner, smaller, and lighter. In order to satisfy this need, a semiconductor chip to be incorporated therein must be thin. Although the thickness of a wafer which forms a semiconductor chip is 125-150 μm at present, it is expected that the thickness of a semiconductor wafer must be 25-50 μm for a next generation chip.
A conventional method for thinning a semiconductor wafer is shown in FIG. 6. Specifically, a protecting tape is attached to a surface (A-surface) of a semiconductor wafer on which a circuit (element) has been formed. The wafer is turned over, and the rear surface (B-surface) of the semiconductor wafer is ground by a grinder so as to make the wafer thin. The rear surface of the semiconductor wafer which has been thinned is fixed onto a dicing tape retained by a dicing frame, and the protecting tape covering the surface (A-surface) of the semiconductor wafer on which a circuit (element) has been formed is stripped in this state. Next, the wafer is cut into each chip by a dicing device.
The above-mentioned method has been disclosed in Document 1. According to Document 1, the protecting tape having heat resistance is stripped by using a strong adhesive tape which is bonded to one end of the protecting tape.
Document 2 has disclosed that a protecting base obtained by immersing ladder-type silicone oligomer in an aluminum nitride—boron nitride porous sintered material is used instead of a protecting tape, and the protecting base and a semiconductor wafer are bonded together by using a thermoplastic film.
Document 3 has disclosed that a protecting base is made of a material having the same thermal expansion coefficient as a semiconductor wafer such as alumina, aluminum nitride, boron nitride, or silicon carbide, and a thermoplastic resin such as polyimide is used as an adhesive for bonding the protecting base and the semiconductor wafer together. As a method for applying the adhesive, Document 3 has proposed a method in which the adhesive is formed into a film having a thickness of 10-100 μm, or an adhesive resin solution is applied by spin coating and dried so as to form a film having a thickness of 20 μm or less.                [Document 1] Japanese Patent Application Publication No. 2002-270676, paragraph 0035        [Document 2] Japanese Patent Application Publication No. 2002-203821, paragraph 0018        [Document 3] Japanese Patent Application Publication No. 2001-77304, paragraphs 0010 and 0017        
In the case of using the protecting tape as disclosed in Document 1, cracking or chipping of the semiconductor wafer easily occur when the tape is stripped. Also, the protecting tape is not sufficient to support the thinned semiconductor wafer. Consequently, transfer needs to be conducted by manpower, and automation is impossible.
In the techniques of Documents 2 and 3, handling or transfer can be automated by using the protecting base (supporting plate) which is made of alumina, aluminum nitride, boron nitride, or silicon carbide instead of a protecting tape. However, since a thermoplastic film which has been dried is used as an adhesive means for bonding the protecting base and the semiconductor wafer together, a heating step is required for softening the thermoplastic film. Also, since non-uniformity of the adhesion strength locally occurs in the case of using a film-shaped adhesive, there are drawbacks that the adhesive unpreferably strips at the time of grinding, or some areas of the adhesive won't strip at the time of dicing.